sock_udp: fix doc of sock objects
sock_udp: fix doc of sock objects

jasonatran/RIOT,OlegHahm/RIOT,yogo1212/RIOT,yogo1212/RIOT,jasonatran/RIOT,OlegHahm/RIOT,basilfx/RIOT,kaspar030/RIOT,basilfx/RIOT,basilfx/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,basilfx/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kYc0o/RIOT,kYc0o/RIOT,basilfx/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,jasonatran/RIOT,yogo1212/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,yogo1212/RIOT